The present invention relates to a conveyance system for conveying cargoes in a plant or warehouse and particularly to a conveyance system in which carrier vehicles run along a track(s) provided on the floor or the ceiling for conveying cargoes.
A conventional conveyance system in which a plurality of carrier vehicles run on a track(s) for conveying cargoes is disclosed in Japanese Unexamined Patent Publication No. 2000-353015 (referred to as Prior Art 1 hereinafter). The conventional conveyance system of Prior Art 1 will be explained referring to FIG. 13.
The carrier vehicles 106 (106a, 106b, 106c, . . . ) run on a track 102 provided on the floor or the ceiling to convey cargoes to and from warehouses 101 (101a, 101b, 101c, . . . ). There are stations 103 (103a, 103b, 103c, . . . ), each for one of the warehouses, where the carrier vehicles 106 stop to load and unload their cargoes. The track 102 has switches to form double-track sections where the stations 103 are sited. This allows the carrier vehicles to run continuously but not to be interrupted by any other carrier vehicle which stops to load or unload its cargo.
In the conveyance system of Prior Art 1, the carrier vehicle having unloaded its cargo stands by at the station before it receives a command. For example, as shown in FIG. 13, the carrier vehicle 106b has unloaded its cargo for storage in the warehouse 101a 1and stands by at the station 103a before it receives another command.
If the other carrier vehicle 106a arrives for storage in or picking up from the warehouse 101a, the vehicle 106b has to be moved from the station 103a. The conveyance system of Prior Art 1 has a communication point 107a provided on the track 102 for detecting the passage of the carrier vehicle 106a and when finding that the destination of the carrier vehicle 106a is the station 103a, commanding the carrier vehicle 106b to move from the station 103a to another free station (e.g. 103b).
Another conventional conveyance system is disclosed in Japanese Patent Unexamined Publication No. Sho. 59-180610 (referred to as Prior Art 2 hereinafter) where a plurality of carrier vehicles run on a track(s) to convey cargoes. The conventional conveyance system of Prior Art 2 will be explained referring to FIG. 14.
In the conveyance system of Prior Art 2, a carrier vehicle 106 runs on a track 102 to convey cargoes between stations 103.
It is now assumed that the carrier vehicle 106 arrives at the station 103 which is busy or out of order, thus allowing no unloading. Then, the carrier vehicle passes the station and loops the track before coming back again. If the station fails to eliminate its busy or out-of-order condition by then, it forces the carrier vehicle to loop the track once more without unloading its cargo.
When such a condition remains for a considerable length of time, the carrier vehicle has to run on the track continuously and its operating efficiency declines. For compensation, the conventional conveyance system of Prior Art 2 has a transmitter 108 provided on the track 102 for supplying each carrier vehicle with a signal upon its passing by to count number of passages. When the number of passages exceeds a given number, the carrier vehicle 106 is commanded to move to and stay at an assigned station 105.
If too many carrier vehicles are on the track as compared with the number of cargoes to be conveyed, they might undesirably meet each other at the switches or stations, increasing the standby duration and interrupting the smoothness of conveying actions. Also, the carrier vehicles run without cargoes more frequently and operating efficiency declines. If the carrier vehicles are too few on the track as compared with the number of cargoes to be conveyed, waiting for the carrier vehicles will be extended, interrupting the smoothness of conveying actions.
It is therefore desired to determine the number of the carrier vehicles depending on the number of cargoes to be conveyed or the total load to be handled by the conveyance system. However, the number of cargoes is not constant but usually varied time to time. The conventional conveyance system of Prior Art 1 or 2 has a predetermined number of the carrier vehicles provided for use regardless of the total load of cargo to be conveyed and its smooth, efficient conveying action might be interrupted by an abrupt change in the load.
It is hence an object of the present invention to provide a conveyance system which can automatically control the number of carrier vehicles corresponding to a change in the load to be handled.
For achievement of the object of the present invention, the conveyance system which performs conveying action with operating a part of or all of a plurality of carrier vehicles includes a loop track, a plurality of carrier vehicles which run on the track, a station where each carrier vehicle performs loading and unloading of a cargo, and a standby station for storing the carrier vehicles at their standby state. The number of the carrier vehicles to be used is determined from a conveyance command number.
A method of controlling conveyance system according to the present invention includes the steps of: counting the number of conveying actions either in operation and to be carried out; calculating an optimum number of the carrier vehicles corresponding to the number of conveying actions; counting the number of the carrier vehicles at their in-action state; and when the number of the carrier vehicles at their in-action state is smaller than the optimum number, using some carrier vehicles at their standby state.
Another method also includes the steps of: counting the number of conveying actions either in operation and to be carried out; calculating an optimum number of the carrier vehicles corresponding to the number of conveying actions; counting the number of the carrier vehicles at their in-action state; and when the number of the carrier vehicles at their in-action state is greater than the optimum number, shifting some of the carrier vehicles from the in-action state to the standby state.